Method for controlling the temperature of an appliance

ABSTRACT

A method of controlling the temperature of a food preparation or sanitation appliance includes sensing the actual temperature of the appliance. When the temperature of the appliance is greater than a maximum threshold temperature, a compressor which supplies pressurized air to a burner is shut-off and a flame sensor, which senses the presence or absence of a flame, is overridden so the appliance acts as if a flame is present. When the actual temperature is less than a minimum threshold temperature, the compressor is started.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional application61/027,748 filed in the United States Patent and Trademark Office onFeb. 11, 2008, the complete disclosure of which is incorporated hereinby reference and priority to which is claimed.

BACKGROUND OF THE INVENTION

This invention relates to a method for controlling the temperature of anappliance and, in particular, to a method of controlling the temperatureof an appliance which is heated by a burner.

In food preparation and sanitation applications it is desirable tomaintain the temperature of the appliance constant. For appliancesheated by a burner this typically requires a presence of a user tomanually adjust the output of the burner though the rotation of acontrol knob. This may be labor intensive and inconvenient. To eliminatethe need for manual adjustment to the output of the burner, someappliances are provided with thermostatically controlled fuel valveswhich maintain or restrict the flow of fuel to the burner based on thetemperature of the appliance. However, these appliance may also requirea constant flow of fuel to the burner to maintain a flame. This is bothexpensive and wasteful.

To conserve fuel some appliances operate on a flame on/flame out basis.When the temperature of the appliance is greater than a maximumthreshold temperature the flame is extinguished. When the temperature ofthe appliance is less than a minimum threshold temperature the flame isignited. For example, in United States Patent Application PublicationNo. US 2006/0172238 to Cook, there is disclosed a method for controllinga heater which comprises turning a pilotless igniter on for a firstperiod of time and opening a fuel source valve whenever the temperatureis less than or equal to a low temperature setting, and closing the fuelsource valve whenever the temperature is greater than or equal to a hightemperature setting or a flame sensor indicates that the flame has goneout.

Since the method disclosed by Cook is, at least in part, dependent onthe presence or absence of a flame it may be limiting because theabsence of a flame when the temperature of the heater to greater thanthe low temperature setting, but less than the high temperature setting,may result in the heater being shut down. There is therefore a need foran improved method of controlling the temperature of an appliance.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodfor controlling the temperature of an appliance which is heated by aburner.

It is a particular object of the present invention to provide animproved method of controlling the temperature of an appliance which isresponsive only to the temperature of the appliance.

There is accordingly provided a method of controlling the temperature ofa food preparation or sanitation appliance which includes sensing theactual temperature of the appliance. When the temperature of theappliance is greater than a maximum threshold temperature, a compressorwhich supplies pressurized air to a burner is shut-off and a flamesensor, which senses the presence or absence of a flame, is overriddenso the appliance acts as if a flame is present. When the actualtemperature is less than a minimum threshold temperature, the compressoris started.

Preferably a set temperature point is inputted into the appliance. Theminimum threshold temperature is less than the set temperature point andthe maximum threshold temperature is greater than the set temperaturepoint. In one example, the minimum threshold temperature is between 1°F. and 10° F. less than the set temperature point, and the maximumthreshold temperature is between 1° F. and 10° F. greater than the settemperature point. In a preferred example, the minimum thresholdtemperature is 3° F. less than the set temperature point, and themaximum threshold temperature is 3° F. greater than the set temperaturepoint.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings:

FIG. 1 is an front elevation view of a thermostatic control deviceinterfaced with a stove;

FIG. 2 is an isometric view of the stove of FIG. 1;

FIG. 3 is an isometric, exploded view of a portion of a fuel deliveryassembly of the stove of FIG. 1;

FIG. 4 is a continuation of FIG. 3 showing a flame sensor, needle valve,and air valve of the fuel delivery assembly;

FIG. 5 is an isometric view of the thermostatic control device of FIG.1;

FIG. 6 is a block diagram illustrating the interface between thethermostatic control device and stove and FIG. 1;

FIG. 7 is a flow chart illustrating the logic of the thermostaticcontrol device; and

FIG. 8 is a state diagram illustrating the overall functionality of thethermostatic control device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings and first to FIG. 1, this shows a stove 10 anda thermostatic control device 50. The stove 10 is similar to the typedisclosed in U.S. Pat. No. 6,450,801 to Wilnecheko et al., the completedisclosure of which is incorporated herein by reference. The stove 10provides heat for a tray ration heater 13. In this example, the stove 10and the tray ration heater 13 together are considered to be a singleappliance 11. However, in other examples, the stove itself may beconsidered an appliance.

As best shown in FIG. 2, the stove 10 comprises a burner unit 12 whichincludes an air aspiration, infrared burner 14 similar to the typedisclosed in U.S. Pat. No. 5,527,180 to Robinson et al., the completedisclosure of which is also incorporated herein by reference. The burner14 utilizes a two-stage combustion process which re-circulatescombustion gases to achieve complete fuel vaporization. Byre-circulating the combustion products and re-burning the gases within aburner head 16, the two-stage process burns virtually all of thehydrocarbons, approaching complete combustion. The benefits of acomplete combustion are numerous, including maximum efficiency and heattransfer, little to no smoke, and very low carbon monoxide emissions.When multiple stove units are used in an enclosed kitchen, near completecombustion is necessary to ensure the health and safety of cookingstaff. The infrared burner technology helps preserve clean breathablekitchen air.

The burner 14 is supplied with fuel by fuel delivery assembly 15 whichis shown in greater detail in FIG. 3. The fuel delivery assembly 15comprises a fuel block 17 and an air atomizing type nozzle 18. Thepurpose of the nozzle 18 is to transform liquid fuel into a finelyatomized spray with increased surface area to promote the mixing of airand fuel, and facilitate evaporation. In this example, the nozzle 18 isa twin fluid air-assist type nozzle, also known as a siphon type airatomizing nozzle, in which the introduction of high velocity air into aslow-moving fuel causes the fuel to be disintegrated by mechanicalenergy from the air. Referring back to FIG. 2, a compressor 20 suppliespressurized air to the nozzle 18, shown in FIG. 3, through an airconduit 23. This creates a low pressure area inside the nozzle cavity.The low pressure is used to siphon fuel from a fuel container 24,through a fuel regulator 26 and a siphoning fuel line 28, to the nozzle18. The nozzle 18 provides an atomized fuel spray which is ignitedwithin a combustion chamber of the burner unit 12. A control knob 30 isused to regulate both the output of the compressor 20 and the flow offuel to the nozzle 18.

As best shown in FIG. 4, the control knob 30 is coupled to both afeedback potentiometer 32 and a needle valve 34. The control knob 30engages slots 31 and 33 on the feedback potentiometer 32. The controlknob 30 also engages a cap 35 of the needle valve 34. The feedbackpotentiometer 32 controls the output of the compressor 20 andaccordingly the air pressure in the nozzle 18. The needle valve 34 hasmale threading 36 which engages corresponding female threading (notshown) in the fuel block 17 which is shown in FIG. 3. As the stove 10operates, fuel is siphoned from the fuel container 24 through the fuelregulator 26 and the fuel line 28, all of which are shown in FIG. 2.Fuel then flows from the fuel line 28 through a fuel valve 38 which, inthis example, is a shut off solenoid valve shown in FIG. 3. Finally fuelflows from through the fuel valve 38 to the needle valve 34, shown inFIG. 4, and to the nozzle 18. Rotation of the control knob 30 causesaxial movement of a valve stem (not shown) of the needle valve 34 whichcontrols the flow of fuel to the nozzle 18. At the same time, thecontrol knob 30 also rotates the feedback potentiometer 32 to controlthe speed of the compressor so that the amount of pressurized airsupplied to the nozzle 18 is correct for the amount of fuel beingsupplied through the fuel line 28. Accordingly, when the control knob 30is rotated, two fuel flow parameters are altered simultaneously. Thesetwo fuel control parameters are arranged so that a linear fuel rate ismaintained.

An igniter 44, shown in FIG. 3, is mounted on the fuel block 17. Theigniter 44 ignites the air fuel mixture expelled from the nozzle 18. Theigniter 44 is attached to the fuel block 17 so its position relative tothe nozzle 18 and fuel spray is maintained. This ensures reliable flameignition and longer igniter life. A flame sensor 40, shown in FIG. 4, isalso mounted on the fuel block 17. The flame sensor 40 views a flame inthe burner 12 through an aperture 42, shown in FIG. 3, in the fuel block17. The flame sensor 40 is located immediately beside the nozzle 18,optimizing the viewing angle. The view of the flame sensor 40 isgenerally parallel to a longitudinal axis of nozzle 18.

The flame sensor 40 measures the flame intensity and converts it to a DCvoltage signal. The flame sensor 40 is designed to respond to changinglight intensity rather than responding to steady-state light intensity.All flames have a characteristic changing light intensity or flickerfrequency. The flame sensor 40 is tuned to respond to a specific flameflicker frequency and ignore other sources of light intensities such assunlight or fluorescent light. The flame sensor 40 measures the flameintensity in the infrared spectrum. A threshold value for the DC signalis established. If the DC signal is over the threshold a flame ispresent. If the DC signal is below the threshold no flame is present.When no flame is present the flame sensor 40 signals a stove controller27, shown in FIG. 6, to shut down the stove 10. The stove controller 27is in the form of a micro-processor and controls all electrical andprocess functions of the stove 10. The stove controller 27 controls andmodulates the input voltage and output voltage as constant voltagecomponents using PWM control. In this example, each of the outputcircuits utilizes MOSFET style transistors with internal thermalprotection to control the required devices.

The fuel delivery assembly 15, as shown in FIG. 3, further includes anair valve 37 which in this example is a three-way, two position solenoidvalve. The air valve 37 selects between an air filter 39 and a ventassembly (not shown). This allows the compressor 20 to operate in twomodes. In a first mode, the air valve 37 is in a normal position, andair is drawn through the air filter 39 allowing the compressor 20 tosupply pressurized air to the nozzle 18. In a second mode, the air valve37 is in an energized position, and air is drawn from the vent assemblyto assist in re-fueling the fuel container 24 by drawing air from thefuel container 24 and drawing fuel into the fuel container 24 through afuel supply line (not shown). Referring to FIG. 6, the vent assemblyalso includes a vent valve 41 for the fuel container 24 which, in thisexample, is a shut off solenoid valve. The vent valve 41 is closedduring re-fueling and when the stove 10 is non-operational to create alow pressure area in the fuel container 24. When the stove 10 isoperational the vent valve 41 is open to maintain the fuel atatmospheric pressure. There is also a fuel level sensor 35, also shownin FIG. 6, which signals the stove controller to stop the compressor 20when the fuel container 24 is full. The fuel level sensor may alsosignal the stove controller when the fuel container 24 is empty.

The thermostatic control device 50 is best shown in FIG. 5 and generallycomprises a control box 52, a temperature sensor 54, and a wiringharness 56. In this example, the control box 52 has an input mechanismincluding two input keys 58 and 60 which allow for the adjustment of aset temperature point. One input key 58 is used to increase the settemperature point and the other input key 60 is used to decrease the settemperature point. The set temperature point may be any temperaturevalue and, preferably, between a minimum temperature of 100° F. and amaximum temperature of 500° F. The control box 52 also includes twoindicators 62 and 64. Indicator 62 is a green RUN indicator andsignifies that the thermostatic control device 50 is operating in atemperature control mode. Indicator 64 is a red SERVICE indicator 64 andsignifies that a fault has occurred. There is an easily readablealpha-numeric display 66 disposed on a front of the thermostatic controldevice 50. In this example, the display 66 is a bright LCD upon whichboth the actual temperature of the appliance and set temperature pointare displayed as shown. The actual temperature of the appliance is shownlarger than the set temperature point and is located near the center ofthe display 66 in the illustrated example. The set temperature point islocated in an upper right hand corner of the display 66 in theillustrated example. The display 66 may also relay output regarding theoperation of the thermostatic control device 50. In other examples thedisplay could vary.

The temperature sensor 54 is operatively connected by a cable 55 to asealed connector (not shown) on a back of the thermostatic controldevice 50. Preferably, the temperature sensor 54 is able to be connectedto and disconnected from the control box 52 without the use of tools. Inthis example, the temperature sensor 54 is a direct-connect temperatureprobe. However, any suitable temperature sensor may be used. The harness56 is also sealedly connected to the back of the control box 52 and,preferably, permanently connected to the back the back of the controlbox 52. A distal end 70 of the harness 56 has a sufficient amount andstyle of connectors 70 a, 70 b, and 70 c to properly interface withvarious food preparation or sanitation appliances. The harness 56 mayalso be provided with a protective sheath to protect the wiring fromdirt, food and other miscellaneous debris. In this example, the harness56 interfaces the thermostatic control device 50 with the stove 10 asshown in FIG. 1. However, any other suitable means such as wirelesscommunications may be used to interface the thermostatic control devicewith an appliance.

In operation, and as best shown in FIG. 1, the harness 56 operativelyconnects the thermostatic control device 50 to a control panel 13 of thestove 10. The stove 10 provides a power source for the thermostaticcontrol device 50. In this example, the control panel 13 is hingedlyconnected to the stove 10 and may be opened as shown in FIG. 2. Thecontrol panel 13 also houses the stove controller 27 which is shown inFIG. 6. The temperature sensor 54 is retained by a bracket 72 mounted onthe appliance 11 and is able to sense the actual temperature of theappliance 11. When the stove 10 is powered-up the thermostatic controldevice 50 may also be powered-up. Alternatively, when the thermostaticcontrol device 50 is powered-up the stove 10 may also be powered-up. Auser may enter a desired set temperature point into the thermostaticcontrol device 50 using the input keys 58 and 60. The thermostaticcontrol device 50 then operates the stove 10 in a flame on/flame outfashion to maintain the actual temperature of the appliance 11 at orabout the set temperature point. The thermostatic control device 50 mayalso be provided with a mounting bracket to allow for mounting of thethermostatic control device 50 on the stove 10, the appliance 11, oranother support structure.

Referring now to FIG. 6, this is a block diagram illustrating theinterface between the thermostatic control device 50 and stove 10. Thethermostatic control device 50 is connected in-line between the stove 10and select stove devices such that the thermostatic control device 50can monitor and manipulate the compressor 20, fuel valve 38, flamesensor 40, and igniter 44. The thermostatic control device 50 has theability to interrupt or switch off the compressor 20 and/or fuel valve38 outputs. Thermostatic control device 50 also has the ability todivert compressor output to the igniter 44. Finally, the thermostaticcontrol device has the ability to drive a high analog voltage level suchthat a flame on signal is simulated or, alternatively, drive a lowanalog voltage level such that a flame out signal is simulated.

Referring to FIGS. 1, 2 and 6, when the actual temperature of theappliance 11 equals or exceeds a maximum threshold temperature, thetemperature sensor 54 is in communication with and signals athermostatic control device controller 53 which is disposed in thecontrol box 52. The thermostatic control device controller 53 in turndiverts compressor output power to the ignitor 44 thereby interruptingthe compressor 20. As a result, the compressor 20 is shut-off and theflame is extinguished because there is no low-pressure area to siphonfuel from the fuel tank 24 to the nozzle 18. The thermostatic controldevice controller 53 also drives a signal from the flame sensor 40 to ahigh analog voltage to simulate the presence of a flame so that thestove controller 27 acts as if a flame is still present. Accordingly,only the compressor 20 is shut-off and not the entire stove 10. Thestove 10 is thereby maintained in a standby state. When the actualtemperature of the appliance 11 is equal to or less than a minimumthreshold temperature, the temperature sensor 54 signals thethermostatic control device controller 53. The thermostatic controldevice controller 53 reconnects the compressor output to the compressor20 and drives a signal from the flame sensor 40 to a low analog voltageto simulate the absence of a flame. This causes the stove controller 27to re-ignite the flame. As a result, fuel flows to the nozzle 18 and theflame is re-ignited. The stove 10 is thereby returned to a fullyoperational state. This cycle, shown in FIG. 7, is repeated as theactual temperature of the appliance 11 fluctuates between the maximumthreshold temperature and the minimum threshold temperature.

The maximum threshold temperature is a temperature value which isgreater than the set temperature point and, in this example, preferably3° F. above the set temperature point. The minimum threshold temperatureis a temperature value which is less than the set temperature point and,in this example, preferably 3° F. below the set temperature point.Accordingly, the thermostatic control device 50 is able to maintain theactual temperature of the appliance 11 at or about the set temperaturepoint without requiring manual and intermittent rotation of the controlknob 30. However, the thermostatic control device 50 may also have amanual setting in which the output of the burner unit 12 may becontrolled through rotation of the control knob 30. The thermostaticcontrol device 50 offers the further advantage that it does notinterfere with control of the stove's vent valve 41, air valve 37, orfuel level sensor 35, thereby allowing for normal re-fueling of thestove as described herein.

Referring to FIGS. 1, 6, and 8 the functionality of the thermostaticcontrol device 50 will now be discussed. FIG. 8 is a state diagramillustrating the overall functionality of the thermostatic controldevice 50. Initially the stove 10 is powered up in a normal fashion to aPOWERUP state as indicated by block 100. The stove 10 is then maintainedin a STANDBY state, as indicated by blocks 110 and 130, while thethermostatic control device 50 monitors the actual temperature of theappliance 11 and stove outputs. If a fault is detected the stove 10 issent to a SHUT DOWN state as indicated by block 150. Otherwise thethermostatic control unit completes a pre-check as indicated by block140. If the actual temperature of the appliance 11 is below the settemperature point, the thermostatic control device 50 leaves all inputsand outputs unaltered, allowing the stove 10 to continue to provide heatin a HEAT state as indicated by block 160. If the actual temperature ofthe appliance 11 is equal to or exceeds the maximum thresholdtemperature, the thermostatic control device 50 prevents the burner unit14 from igniting and goes straight to a COOL state indicated by block180.

When the actual temperature of the appliance 11 is initially below theset temperature point, the stove 10 operates in the HEAT state indicatedby block 60 to raise the temperature of the appliance 11. Once theactual temperature of the appliance 11 is equal to or exceeds themaximum threshold temperature, the thermostatic control device 50 shutsoff the burner unit 12 by diverting compressor output power to theigniter 44. This provides a load to the compressor drive circuit andprevents an open circuit fault condition. The thermostatic controldevice 50 then drives the flame sensor signal to a high analog voltageto simulate the presence of a flame. This results in a SIMULATE state asindicated by block 170. Power to the compressor 20 and fuel valve 38 isthen interrupted to halt operation of the burner unit 12 resulting inthe COOL state indicated by block 180.

In the COOL state 180 the actual temperature of the appliance 11 willdecrease. Once the actual temperature of the appliance 11 is equal to orless than the minimum threshold temperature the thermostatic controldevice 50 restarts the stove 10 as indicated by block 190. In theRESTART state, the thermostatic control device drives the flame sensor40 to a low analog voltage level to stimulate the absence of a flamethereby triggering the stove to re-ignite the burner unit 12. This isdone by temporarily turning off the fuel valve 38 and energizing theigniter 44. After a short delay fuel is re-introduced to the burner unit12 by turning on the fuel valve 38. Reconnecting the compressor and fuelvalve outputs allows the stove controller 27 to control the compressor20 and fuel valve 38 resulting in an ENABLE STOVE state indicated byblock 200. The thermostatic control device 50 then releases the flamesensor signal so the presence of a flame is reported to the stovecontroller 27. At this point the stove controller 27 re-ignites theburner 14. The thermostatic control unit 50 now stops divertingcompressor output power to the igniter 44 resulting the HEAT stateindicated by block 160. The stove 10 remains in the HEAT state 160 untilthe actual temperature of the appliance 11 is equal to or greater thanthe maximum threshold temperature at which point the cycle repeats. Ifat any point in the cycle a fault is detected a SHUT DOWN STOVE state,indicated by block 210, is triggered.

It will be understood by a person skilled in the art that although inthe example provided above the thermostatic control device is interfacedwith a stove, that in other examples the thermostatic control device maybe interfaced with any appliance comprising a burner. Alternatively, thethermostatic control device may be integral with any appliancecomprising a burner.

It will be further understood by someone skilled in the art that many ofthe details provided above are by way of example only and are notintended to limit the scope of the invention which is to be determinedwith reference to the following claims.

1. A method of controlling the temperature of a food preparation orsanitation appliance in which a compressor supplies pressurized air to aburner and a flame sensor senses the presence or absence of a flame, themethod comprising: sensing the actual temperature of the appliance;shutting-off the compressor and overriding the flame sensor so theappliance acts as if a flame is present whenever the actual temperatureof the appliance greater than a maximum temperature threshold; andstarting-up the compressor whenever the actual temperature of applianceis less than a minimum threshold temperature.
 2. The method as claimedin claim 1 further including inputting a set temperature point into theappliance, the minimum threshold temperature being less than the settemperature point and the maximum threshold temperature being greaterthan the set temperature point.
 3. The method as claimed in claim 1further including inputting a set temperature point into the appliance,the minimum threshold temperature being between 1° F. and 10° F. lessthan the set temperature point, and the maximum threshold temperaturebeing between 1° F. and 10° F. greater than the set temperature point.4. The method as claimed in claim 1 further including inputting a settemperature point into the appliance, the minimum threshold temperaturebeing 3° F. less than the set temperature point, and the maximumthreshold temperature being 3° F. greater than the set temperaturepoint.
 5. A method of controlling the temperature of a food preparationor sanitation appliance in which a compressor supplies pressurized airto a burner and a flame sensor senses the presence or absence of aflame, the method comprising: sensing the actual temperature of theappliance; diverting compressor output power to an ignitor and driving asignal from the flame sensor to a high analog voltage to stimulate thepresence of a flame when the temperature sensor senses that the actualtemperature of the appliance is greater than a maximum thresholdtemperature; reconnecting compressor output power to the compressor anddriving a signal from the flame sensor to a low analog voltage tostimulate the absence of a flame when the temperature sensor senses thatthe actual temperature of appliance is less than a minimum thresholdtemperature.